The invention relates to a measure transformer for detecting hydrocarbons in gases.
Increasingly strict environmental legislation is forcing automobile manufacturers to develop and use exhaust emission control systems, usually catalytic converters, with ever better conversion rates in order to maintain the government-specified maximum values of emitted emissions such as nitrogen oxides, carbon monoxide, or unburned hydrocarbons. At the same time, it is required that the function of the exhaust emission control systems be monitored continuously during operation and that defective function be indicated, ranging from exceeding the limits as a result of aging phenomena of the catalyst to a total failure of the xcex-probe that controls the combustion stoichiometry. For this so-called on-board diagnosis (OBD), an exhaust sensor located downstream of the exhaust emission control system is required that monitors the function of the exhaust emission control system during operation and whose sensor signal serves as a basis for determining the state of the exhaust emission control system.
With a four-stroke engine operated at xcex=1 (1 represents the fuel-air mixture), the emissions are drastically reduced by a three-way catalyst. While it is not difficult to meet the nitrogen oxide and carbon monoxide limits, theoretically unburned hydrocarbons (HC) pose the greatest problems. Malfunction of the exhaust emission control system is indicated only when the HC concentration in the exhaust increases.
There are many ways to diagnose an exhaust emission control system. Several patents such as DE 34 13 760, U.S. Pat. No. 5,740,676, U.S. Pat. No. 5,467,594, and DE 42 09 136 for example as well as the literature references [1] and [2] cited as examples propose providing xcex-probes upstream and downstream of the catalytic converter. The oxygen storage capacity and hence indirectly the function of the catalytic converter can be determined from a comparison of the amplitude fluctuations in the probe output signals upstream and downstream of the catalytic converter. Such methods are already used in mass production. Another frequently discussed method is diagnosis of the exhaust emission control system by means of one or more temperature sensors. In this case, it is the reaction heat resulting from the conversion of the hydrocarbons in the untreated exhaust that is detected. Examples will be found in [3]-[7] or in DE 42 01 136. Direct determination of the hydrocarbon concentration in the purified exhaust by means of an HC sensor would be much simpler and more precise than determination of values that depend only indirectly on emissions.
Such direct HC sensors can incorporate for example HC measurement by means of a surface ionization detector [8], but this method depends to a significant degree on the gas throughput, the type of hydrocarbons, and the oxygen content of the exhaust.
Another type of HC sensor is the familiar catalytic sensor (also known as the pellistor) described here using the example in EP 0 608 122. For such sensors, oxygen is always necessary to burn the hydrocarbons so that the output signal depends largely upon the oxygen content of the exhaust. In addition, very exact temperature control and measurement are required since the electrical resistance of a temperature-dependent part is measured. Therefore, such a sensor principle is unsuited for use in the exhaust line.
A sensor design that consists of an oxygen generator, oxygen diffusion zone, HC sensor zone, and at least two temperature control zones and is therefore very complex is described in U.S. Pat. No. 5,689,059. This sensor is suitable for exhaust but requires electrical terminals in considerable numbers. In addition, this sensor, which in reality a sensor system, requires very complex and costly control and regulating electronics so that it cannot be used for the broad mass market.
Hydrocarbon sensors using planar technology are less expensive to manufacture.
DE 0 046 989 proposes an HC sensor based on tungsten oxide made by the planar technique which can be used only at room temperature.
Pt-MOSiC sensors based on silicon carbide are proposed in [9] for use in motor vehicles. However, the operating mechanism is not easy to understand and the signals are dependent not only on the hydrocarbon but also on oxygen and temperature. Manufacture of planar structures on SiC is also costly and therefore cannot be used for the motor vehicle mass market.
Widely used, inexpensive sensors are made on a ceramic substrate from SnO2. Examples include EP 0 444 753 or EP 0 603 945. In this sensor principle, the electrical sensor resistance changes with the HC concentration in the gas. Sensors of this type are used in large numbers as sensitive elements in gas warning systems and their functional mechanism is widely known. An attempt to use such sensors in an automobile is described in [10] and [11]. Unfortunately, these sensors lose their gas-sensitive properties at temperatures above several hundred degrees Celsius and change their resistance only with the oxygen partial pressure of the gas. The long-term stability of these sensors is not guaranteed either.
Resistive sensors based on metal oxides which have been proposed more frequently as an oxygen-detecting element but not as an HC sensor are likewise manufactured using planar technology and are suitable for use in exhaust. In the resistive principle, the electrical resistance of the sensitive material is used as a measured value. For example, DE 37 23 051 proposes doped titanates, zirconates, or stanates as resistive oxygen-sensitive materials which are applied according to DE 37 23 052 using thick film technology to a ceramic substrate. DE 42 02 146, DE 42 44 723, and DE 43 25 183 propose compositiones based on cuprates manufactured using thick film technology as oxygen-sensitive materials. DE 44 18 054 mentions lanthanum ferrites doped with alkaline earths for the same purpose. Such multiple metal oxides which are usually present in the perovskite structure have the advantage of increased chemical stability and greater long-term stability over the sensors made of simple metal oxides, TiO2 [10] for example, that have been in use for a long time. All of these oxygen sensors however have a typical temperature curve of the electrical resistance according to an exponential function typical for semiconducting metal oxides, in other words the sensor output signal depends not only on the oxygen partial pressure of the exhaust but on the sensor temperature as well. Hence, for such oxygen exhaust sensors, an exact temperature measurement is linked to a costly electronic regulation or a reference that is not exposed to the exhaust and is kept at a constant temperature must be integrated in the substrate which is also expensive and leads to problems with long-term stability.
The series connection of two resistive oxygen sensors which in addition to their oxygen dependence have a temperature dependence with different temperature coefficients of the specific electrical resistance, is proposed in DE 38 33 295. In addition, a compensating resistance must also be included. However, in this method, despite the high costs, a temperature independence of the sensor resistance can be achieved only in a very narrowly delimited oxygen partial pressure range.
A resistive oxygen sensor that is temperature-independent only at a certain oxygen partial pressure is described in a portion of EP 0 062 994. In DE 19 744 316, an oxygen sensor is proposed made of a material in which the oxygen partial pressure range of temperature independence can be varied by deliberate variation (doping) of the layer material.
Typical HC sensors manufactured using planar technology are characterized by the following typical arrangement. On the underside of an electrically insulating substrate a heater and/or a temperature measuring device in the form of a resistance thermometer is mounted. Then, on the substrate surface, an electrode structure is provided that meets the special requirements and a functional layer is mounted on top of this structure.
In EP 0 426 989 and in [12], the electrode structure has a so-called interdigital capacitor arrangement (IDC). Zeolites are proposed as the functional layer. Dependent on the temperature, the complex electrical resistance of the functional layer varies very selectively in this case with the hydrocarbon concentration in a gas. In [13] Ga2O3 is proposed as the material for a resistive planar HC sensor. However, a significant dependence of the sensor output signal on temperature is reported here as well. This sensor reacts to oxygen [12] especially above 900xc2x0 C.
In another method for producing planar sensors, a ZrO2 layer is applied to the substrate and two different electrodes with different electrical potentials are added. The differential voltage between the two different electrodes is then the measurement signal. This method is described in great detail in [14]. A variation is described in U.S. Pat. No.5,352,353 and DE 41 02 741 and DE 41 09 516. It is readily apparent that the output signal from such sensors depends to a large extent upon temperature and naturally on oxygen partial pressure of the exhaust as well. Additional cross sensitivities to hydrogen for example, are also present.
In DE 42 28 052, a sensor is described that consists of a combination of two individual resistive oxygen sensors, with one of the two sensors being provided with a catalytically active coating. As a result, the incompleteness of the combustion in the engine can be determined from the differential signal of the two sensor elements. Such a sensor has the disadvantage that it is not temperature-compensated, in other words as in all resistive oxygen sensors, the output signal is primarily temperature-dependent and depends only secondarily on gas concentration.
To remedy this situation, DE 195 31 202 proposes a bridge arrangement consisting of at least four individual sensor elements of which two arexe2x80x94and two are p-conducting, arranged so that one branch of the bridge is activated catalytically and that anxe2x80x94and a p-conducting sensor are arranged in each branch of the bridge circuit. In addition to the considerable cost and technical problems involved, in applying at least four different materials in addition to the electrical terminals in layers on a substrate compatibly with one another, this system does not produce a sensor that is temperature-independent for all oxygen partial pressure ranges.
The disadvantage of all planar HC sensors is the considerable temperature dependence of the sensor output signal which requires either exact temperature regulation or exact temperature measurement followed by electronic compensation of the signal or the materials that differ only by a cumbersome arrangement of different materials on a support can be partially compensated.
Hence, the goal of the present invention is to provide a measuring transformer for HC detection which eliminates the stated disadvantages of the prior art, especially temperature dependence.
This goal is achieved by the measuring transformer according to claim 1. Advantageous embodiments of the invention as well as special applications of the measuring transformer according to the invention are the subject of additional claims.
According to the invention, two resistive oxygen sensors are provided, one of them provided with a catalytically active layer for reducing hydrocarbons. Both oxygen sensors are characterized by an essentially temperature-independent characteristic. The electrical resistances of the two sensors are measured and both the hydrocarbon concentration and the oxygen partial pressure of the gas to be analyzed can be determined from the two measured values. Since the oxygen partial pressure is measured as well, the available oxygen dependence of the catalytically non-activated oxygen sensor can be calculated.
Both oxygen sensors are held at the same working temperature. As a result of the temperature independence of the sensor output signal, only limited requirements need to be imposed on temperature measurement and regulation. Depending on the desired accuracy, they can even be eliminated. A resistance heater can be provided as a heater for example. It is also possible to provide heating by hot gases which are available in the vicinity of the hydrocarbon detection, for example the exhaust gases from a furnace or an internal combustion engine.
The measurement transformer according to the invention is preferably made using thick film or thin film technology with the two oxygen sensors being mounted on an electrically insulating substrate.
Preferred areas of application for the measurement transformer according to the invention are:
Measurement of the hydrocarbon concentration in the exhaust from a furnace or heating system. In addition, the output signal from the non-activated resistive oxygen sensor can be used to detect the oxygen content in the exhaust from the furnace or heating system.
Measurement of the concentration of flammable gases, especially hydrocarbons, in the ambient air.
Measurement of the hydrocarbon concentration in the exhaust from an internal combustion engine. The output signal of the activated resistive oxygen sensor can be used in addition for detecting the fuel-air mixture in the exhaust from the engine.
Diagnosis of an exhaust emission purification system in the exhaust from an internal combustion engine. In addition, the output signal from the activated resistive oxygen sensor can be used to detect the oxygen-air mixture of the internal combustion engine.
Performing mixture control in an internal combustion engine.
The invention will now be described below using a few preferred embodiments as examples, with reference to drawings. The embodiments described serve only to explain the invention and are not intended to imply that they represent a limitation of the invention to these special embodiments.